Metal membranes for this purpose can lie on a supporting substrate that shall provide for mechanical strength in the composite membrane structure. The invention is particularly directed to manufacturing such a composite membrane structure for use in gas separation by selective diffusion and comprising a metal membrane manufactured on the basis of a method according to the invention.
A number of metals possess perm-selectivity (selective permeability) with respect to various gases. Therefore they can be employed as selective membranes in gas separation. Well known examples of such selective properties are oxygen diffusion in silver and hydrogen diffusion in palladium alloys. A prerequisite for perm-selectivity is that there are no defects in these metal membranes in the form of through holes and cracks that can provide for transport of all types of gases through the membrane by common gas diffusion in the defects.
In order to obtain as high (selective) permeability as possible in the membrane with respect to the gas concerned, it is desirable to make the membranes as thin as possible. Thin metallic films can be produced in a number of well known manners, such as for example sputtering, depositing metal vapor and chemical depositing methods. For mechanical stability such thin films must lie on a mechanically stable, supporting substrate. When employed as membranes for selective separation of gases, the thin metal membrane lies on a supporting substrate having a sufficient permeability for the gas component with respect to which the metal membrane is selective. Supporting porous substrates must have as small pores as possible in order to have the required mechanical strength for the thin metal membrane it supports. Tight substrates must have a high permeability for the gas concerned, and hydrogen is particularly interesting here. This is known in various metals, inter alia, in group V of the periodical system. Previously there have been proposed methods for producing thin (&lt;25 .mu.m) defect-free metallic membranes by sputtering or thermal or chemical depositing on the substrate, i.e. the metal membrane is formed by letting it grow on the substrate until it has the thickness being necessary in order not to contain defects in the form of through holes or cracks. In actual practice it has been found to be difficult to manufacture very thin (&lt;2 .mu.m) defect-free membranes on substrates by means of these methods. These known techniques are described, inter alia, in:
1. Japanese patent publication 4-349926 (A)--patent application No. 3-126105--"Hydrogen gas separation membrane"--Mitsubishi Heavy Ind. Ltd. PA1 2. V. M. Gryaznow, O. S. Serebryannikova, Yu. M. Serov, M. M. Ermilova, A. N. Karavanov, A. P. Mischenko and N. M. Orekhova: "Preparation and catalysis over palladium composite membranes". Applied Catalysts A: General, 96 (1993) 15-23. PA1 3. U.S. Pat. No. 4,857,080 describes the preparation of composite membranes by direct sputtering on a supporting, porous substrate. PA1 4. U.S. Pat. No. 5,409,782 corresponds to some degree to the US patent just referred to, in so far as it relates to methods for forming thin films directly on a substrate. PA1 5. U.S. Pat. No. 4,699,637 describes: First there is prepared a metal membrane of thickness 10-100 microns, this being then put between two metal fabrics for mechanical strength. The actual membrane manufacturing takes place by rolling. PA1 6. D. I. Edlund and W. A. Pledger I. Membrane Science 77 (1993) 255-264 PA1 7. U.S. Pat. No. 5,139,541 (D. I. Edlund 1992).
In an other method there has been employed thicker (25 .mu.m) Pd films being thereafter laid on tight substrates of a metal having a high hydrogen permeability, such as vanadium metal. In order to avoid interdiffusion between the Pd film and the metal substrate, there must be applied a thin oxide layer being porous and having a certain roughness. This method is described in:
Other somewhat remote examples of prior art in this context may be found in U.S. Pat. No. 3,270,381, 4,898,623 and 5,382,344.